Topic: Anyone running 77-78 carbs on and earlier engine?? need some tips!

[CB750R]

I just fired my bike up after a complete rebuild thanks to a broken valve in the engine.... The bike ran carb wise pretty decent before, but there were high dome pistons, now I'm using the totally flat topped ones, and Its kinda half back firring on idle, bogs at high throttle openings, I'm running a4 into one wit a  2.5"D&D cane, with Pods, and 120 jets in the PD50 carbs. it fires up and idles ok but 3/4 to wide open totally bags.  Just wondering if anyone has a good baseline for running the later carbs on an earlier engine.

[scondon]

    I'll just assume the bike you're talking about is the one in your avatar. 77 750F right?  Pistons changed from dome to flat top so I'll also assume that you've changed the head as well(no longer using the 77 F head since it won't work well with flat top pistons)right? And I'll also assume that you've changed carb rubbers or found some way to seal your old ones to the ,now smaller, head intakes.

    I haven't heard of PD50 carbs on any make of 77-78 Honda so maybe you meant PD41A(B) or PD42A(B). If that's the case then 120-125 mains are a good baseline for your set-up, assuming you've already cleaned and synched the carbs and done a full tune-up.

[CB750R]

Actually ya you assumed dead on for all accounts, I have a full 74 engine that some PO tried to put the 77 high dome pistons on, the rubbers for the intakes seem to work ok for the old and the new head both of which have the smaller valves so I know for sure there not 77-78s, your right on my tune up, there's a new dyna, timed perfectly, new plugs, coils check out in there resistance and what not, the carbs were redone prior to my last rebuild, I'm thinkin maybe the idle jets got clogged while sitting, the carbs were rebuilt and driving 34kms before the top end grenaded. so in storage maybe things got gooey, I read a thread of Terry in austrailia who lamented how easy the idle jets clog. I may have to screw around with that and then the needle jets, but I have the right jets in by the sounds of it.

[scondon]

   The idle jets do tend to clog pretty quick on these carbs. I've had them get gunky after just a month and a half of sitting(on the bike with fresh fuel in them).

    if you can, try a couple plug chops, one at 1/2 - 3/4 throttle and one at WOT. Get the bike good and warm by riding to somewhere that you can hold the throttle wide open in 4th gear(preferably a small incline) for 1/2 mile or so. Insert a new plug into one of the holes, at the side of the road, just before you do a run. After you've done a 1/2 mile at WOT, pull in the clutch and kill the engine simultaneously and coast to the side of the road. Pull the new plug and take a look. If it's white then go up one size on your main jet. If it's tan or no real color, then your mains are good and it's time to do the 1/2 - 3/4 throttle plug chop to check if you need to change the needle height. Wet and/or dark and sooty means it's time to drop a size on the mains.

   Anyway, the idea is to check if your main jets are proper size by doing WOT plug chop and then work your way down from there(needle height, then idle mixture). Carb settings can vary a lot between bikes, so jetting and tuning that works great for one 750 doesn't necessarily work for another, even if the bikes are same year and make. That said, I ran 125 mains with the needles raised( 4th notch from the top ) when I had pods and an open exhaust ('78 carbs).

[CB750R]

Thanks for the info man! I'm very glad to hear from another owner with a similar setup, I'd say I'm fairly handy, but I've had extremely good luck with carbs up to this point, so haven't had to master the art of tuning them until now!!! I've got it to idle pretty decently at the moment, so after I get a plate on her I'll go and do a plug chop!! hopefully I'll get a sunny day and send the site some pics of my baby all put back together!!